A multilayer sealing film for a sealing layer is manufactured for example by means of blown film extrusion or flat film extrusion. The film laminate comprising a sealing layer consisting of a sealing film for producing packaging, such as a bag, is generally produced by means of lamination (i.e., bonding by means of an adhesive layer) of a plurality of films. It is likewise known to manufacture the film laminate by means of coextrusion, the multilayer sealing film being coextruded directly from the melt onto a substrate layer of the film laminate. When producing sealing films from blown polyethylene (PE) (typical blown film) or cast polypropylene (PP) (typical cast film), according to the present prior art what are known as slip additives and/or antiblock additives are added. Said additives have the task of making the usually very dull polyolefins (such as PE or PP) smoother, so that said polyolefins can slide better over the metal surfaces of the packaging machines or against themselves during subsequent processing. If this does not occur, undesired plant outages and/or wrinkled sealed seams or leaky packaging may result.
Typically, coefficients of friction (COF) of the sealing layer relative to steel in the range of 0.15 to 0.30, and of the sealing layer with respect to itself in the range of 0.2 to 0.4, are required for processing film laminates of this kind in packaging machines. In particular when processing the film laminates into bags, known as flow packs, in FFS (form fill seal) facilities, the coefficient of friction with respect to steel is a crucial quality feature of a packaging laminate.
The coefficients of friction specified in the present application are determined using the following test specification:
On a test block of dimensions 66×60×16 mm and having a weight of 500 g, a sample of a wrinkle and fold-free sealing film is clamped on one side of the test block (66×60 mm). In this case, the surface of the film to be tested must of course be oriented outwards. For the purpose of clamping, the sample of the film may be larger than the dimensions of the side of the test block. In order to measure the coefficients of friction with respect to steel, the side of the test block on which the film is clamped is placed on a steel table. The test block is then pulled over the steel table and the force required therefor is measured. The coefficient of friction is then determined as the ratio of the measured force and the weight force of the test block (500 g). In order to measure the coefficients of friction of the sealing layer with respect to itself the same procedure is followed, only a wrinkle and fold-free film is also clamped (with the side to be tested facing outwards) on the test bench, on which film the test block is placed. Using a tensile testing machine, the test block is pulled over the base at a constant speed of 150 mm/min over a measuring distance of 50 mm, and the tensile force is measured.
In the process, a distinction is usually made between what is known as the static coefficient of friction, resulting from the maximum force before the test block moves, and what is known as the dynamic coefficient of friction. Said dynamic coefficient of friction results from the almost constant, average force during the constant, smooth movement of the test block. Films that are too dull move only jerkily and therefore cannot be measured, since the forces fluctuate too much. Films of this kind are unusable in practice.
In order to achieve said coefficients of friction, according to the prior art slip additives and/or antiblock additives are added to the sealing film. Antiblock additives are added in large quantities, well in the region of 1000 ppm. Slip additives are used in the sealing film in slip additive concentrations having an S-value of 16,000 to 25,000. The S-value is defined as the product of the thickness of the sealing film and the content of slip additive in ppm (parts per million). In the case of a multilayer film, the S-value of the film is generally calculated by calculating the S-value of each individual layer (content of slip additive in the layer x thickness of the individual layer), and the S-values of the individual layers are added to make the S-value of the film. This is equivalent to determining an average slip additive content of the film in ppm (average of all the layers), weighted according to the layer thicknesses of the layers, and multiplying by the overall thickness of the film.
Oleamides or, now preferably, ecrucamides are generally used as slip additives, which amides migrate outwards, out of the sealing film, over time and settle on the surface of the sealing film and function there as a sliding film. The most significant disadvantage of these products is that said slip additives migrate, as a result of which the following disadvantages may occur:
The sliding friction of the PE or PP sealing film changes, as the temperature increases, owing to improved solubility of the slip additives in the PE or PP, as a result of which the processing conditions of a film laminate comprising a sealing film of this kind as the sealing layer change. This may make it significantly more difficult to process film laminates of this kind (in a packaging machine) or sealing films of this kind (in a lamination process).
The sliding friction changes, following lamination of the film laminate, due to migration of the slip additives out of the sealing film and into the adhesive and/or lamination partner, as a result of which the processing conditions may again change. This may make it significantly more difficult to process film laminates of this kind.
The lamination partner of the sealing film, e.g., BOPET (biaxially-oriented polyethylene terephthalate) or BOPP (biaxially-oriented polypropylene), becomes smoother due to absorbing the slip additive. This may lead to the film laminate no longer being able to be transported in the packaging facility, as a result of which subsequent processing would be impossible.
The migration of the slip additives is also the reason why the S-value for a multilayer film is calculated as set out above. Equalization of the concentrations of slip additive in the individual layers of the sealing film is initiated very quickly due to the migration of the slip additives, and therefore only one S-value can now be assumed for the entire sealing film.
Antiblock additives are usually mineral fillers (for example silicates or talc), the addition of which increases the surface roughness of the sealing film. Although antiblock agents tend not to migrate but cannot sufficiently reduce the coefficients of friction (COF) of the sealing film, and thus the sliding properties, alone. Whereas pure PE has a COF of 0.5 to over 1 (complete blocking), using antiblock additives alone, it is possible to achieve minimal coefficients of friction of 0.3 with respect to steel. However, this is also only the case at high addition concentrations and reduced transparency of the resulting sealing film, which is generally not desirable. Therefore, the addition of slip additives was so far considered necessary in order to achieve the desired low COF. Low concentrations of slip additive can be compensated for by additionally adding antiblock additives in order to nonetheless achieve a low COF. This can be derived from WO 98/37143 A1 for example, in which a multilayer film comprising a slip additive and an antiblock additive is disclosed. A low S-value of the film is compensated for therein by a high concentration of antiblock additive in the region of ≥1000 ppm.
When producing packaging in the form of bags, a film laminate is often folded and welded or sealed to form a bag, as described above. In this case, the film is typically a multilayer laminate, e.g., consisting of a transparent outer layer, e.g., of BOPET (biaxially-oriented polyethylene terephthalate) or BOPP (biaxially-oriented polypropylene), an inner sealing layer made of a sealable polymer in the form of a sealing film as described above, e.g., of PE (polyethylene) or PP (polypropylene), and an optional intermediate barrier layer, e.g., of aluminum of metallized plastics material (e.g., metallized PET). The sealing or welding is typically carried out, in a well-known manner, between temperature-controlled sealing jaws that are pressed together, as a result of which the sealing layer of the film fuses and establishes a connection upon subsequent cooling. In this context, sealable therefore means that the melting temperature of the sealing layer allows for sealing. A wide range of materials are used as the material for the sealing layer, which materials are intended to be meltable and able to be pressed together at typical sealing temperatures of over 100° C. This requirement leads to various mixtures and co-extrudates of LDPE (low density polyethylene), LLDPE (linear low-density polyethylene), EVA (ethylene-vinyl acetate) and similar materials. However, folding the film results in different material thicknesses in the overlap region, which may lead to incomplete sealed seams during sealing, as a result of which the bag produced forms undesired air channels, for example.
This is shown schematically in FIG. 1, on the basis of the example of a bag 1, in this case a vertical tubular bag. The film of the bag 1 is first folded together, lengthwise, into a tube and sealed along the longitudinal seam 2. The tube is sealed at the upper and lower end of the bag 1 by a transverse seam 3 in each case in order to form a bag 1, as a result of which the filling material located therein is enclosed in the bag 1. The overlap region of the two sealed seams, i.e., between the longitudinal seam 2 and the transverse seam 3, is shown enlarged in FIG. 1. Owing to the different material thicknesses along the transverse seam 3, it is possible that the overlapping film 5, in particular in the region of overlapping sealed seams, may not be able to be completely pressed together by the sealing jaws 9a, 9b, as a result of which an air channel 4 may be formed in this region when the transverse seam 3 is sealed, with the result that the bag is leaky. The film 5 is formed in this case as a three-layer laminate comprising an outer BOPET layer 6, an intermediate layer 7 made of aluminum, and an inner sealing layer 8 made of PP. Similar problems also arise in other types of bag, such as cross bottom bags, upright bags, block bottom bags, etc., in the overlap region of a plurality of film layers.
Similar problems may arise when sealing what are known as film-like lids (generally consisting of an aluminum base layer and a sealing layer applied thereto) on the edge of plastics containers, as are conventional in yoghurt packaging, for example. Film-like lids of this kind are generally made of aluminum, plastics material or paper, to which a sealing layer is applied. On account of manufacturing tolerances when producing the plastics containers and/or when producing the film laminates of the lids, differences in thickness may result here too, which differences cannot be compensated for by the pressure of the sealing jaws during sealing, and this may result in the packaging being leaky.
In order to reduce this problem when sealing, special materials for use as a sealing layer have already been developed, but said materials are relatively expensive and are therefore used hesitantly in the packaging industry.
It is not possible to reduce the thickness of the sealing layer, since the sealing layer must have a certain degree of compressibility. In order to be able to make the sealing layer thinner, special polymers are often mixed into the material of the sealing layer, but this again increases the cost of the material.
EP 2 537 770 A1 describes a film material comprising a foamed polymer layer, in particular for producing bags for granular bulk material. The foamed polymer layer is intended to prevent the granular bulk material from showing through at the outer bag surface.
US 2011/0293204 A1 describes a foamed, compressible polymer layer as a sealing layer, in order to improve the sealing properties.
US 2005/0247960 A1 in turn describes a film comprising an embossed sealing layer for forming a bag for vacuum packaging, the embossing forming gaps which form air channels when vacuum packing takes place, through which channels the air can be better suctioned out. A visible pattern, e.g., in the form of letters or any desired shape, may be provided as the embossing. In order for the embossing to be easily visible to the naked eye and in order to ensure the function as an air channel when vacuum packing takes place, the embossing must be relatively deep, generally substantially deeper than 100 μm. The air channels formed must be >˜1 mm wide in order that an expedient volume flow rate for suctioning the air out of the packaging can be achieved.
Embossed sealing layers are also used to prevent cover lids from adhering to one another when stacked one on top of the other, which can lead to problems during processing in processing machines. The embossing creates an air cushion between individual adjacent cover lids, as a result of which the cover lids can be separated easily and reliably. Examples thereof can be found in EP 2 149 447 A1 or WO 2006/096894 A1.